The promoting properties of hard coatings with an amorphous hydrogenated carbon basis to attain dropwise condensation (DWC) of steam on coated copper surfaces were investigated.
Using differently produced coatings, equilibrium contact angles of ties of 65, 74 and 90" could be reached for water. Stable and well reproducible heat transfer measurements could be performed. For a subcooling temperature of the condenser surface of 5 K, the DWC heat transfer coefficient at the vertical wall is 11 times higher for the surface with oes = 90" than that measured for filmwise condensation (FWC), seven times higher for the surface with 0,, = 74" and 3.5 times higher for the surface with B,, = 65". In comparison to the heat transfer coefficient measured for a contact angle of 90" for the heat flux ranging from 0.40.9 MW me2 only 5345%
(for oes = 74") and l-7.5% (for ecq = 65") of the 90"-values were determined. For eeq = 90" the observed DWC keeps very well stable up to a technically achievable maximum heat flux of 1.54 MW m-2. For B.,, = 74" and for peq = 65", however, expanded condensation streams (mixed condensation) appeared on the surface at heat fluxes of 1.03 MW mm2 and 0.7 MW m-2. In these situations the performance characteristic is less developed in comparison to pure DWC, but still better than for pure FWC.